1. Field of the Inventive Concepts
The inventive concepts disclosed generally relate to fluid separators, and more particularly, but not by way of limitation, to four-phase vertical separators for use in oilfield operations.
2. Brief Description of Related Art
Shale gas production operations throughout the United States and all over the world increasingly rely on hydraulic fracturing as a completion process to stimulate natural gas production from shale formations. Hydraulic fracturing involves high-pressure injection of large quantities of water, along with sand, and other small amounts of chemical additives, into a well. The high-pressure water creates small fractures, or cracks, in the surrounding rock formation, and sand or other propant used wedges into the cracks and prevents them from closing up once the water pressure is removed. These cracks allow any gas entrapped in the shale formation to escape and be recovered at the wellhead, and ultimately sold to consumers.
Upon completion of a hydraulic fracturing process in a well, as a result of discontinuing the high-pressure water injection, a large portion of the injected water (from several thousand to millions of gallons) is recovered at the wellhead as flowback water. Such flowback water typically contains some liquid hydrocarbons, some gases dissolved in the water due to the high-pressures in the well, and some solids which may be the propant injected in the well, as well as sand/sediment, and drill cuttings, carried up by the flowback water.
The gases dissolved in the flowback water as a result of the high pressures created during the hydraulic injection are highly flammable and pose serious environmental hazards. The Hazardous Air Pollutants (HAP or HAPs) typically dissolved in or mixed with the flowback water comprise greenhouse gases, volatile organic compounds (VOC or VOCs), such as nitrogen oxides (NOx), methane (CH4), nitrous oxide (N2O), carbon dioxide (CO2), and liquid hydrocarbons, such as formaldehyde, benzene and chlorofluorocarbons (CFCs). VOCs are organic chemicals that have a high vapor pressure at ordinary, room-temperature conditions, and are especially problematic due to their tendency to readily dissolve in or mix with flowback water at high pressures, and to also readily evaporate from the flowback water at atmospheric pressures and ambient temperatures. Further, most VOCs are generally not immediately toxic, but instead have compounding long-term negative health effects on oilfield personnel and other persons and animals exposed to even low concentrations of VOCs for prolonged periods of times.
Due to the health and environmental hazards of HAPs and VOCs, stringent environmental laws and regulations have been enacted on the state (e.g., California Air Resources Board), federal (e.g., EPA, OSHA), and international levels, to minimize the emissions of HAPs, and VOCs in particular, into the environment during oilfield and other industrial and agricultural operations.
Conventional flowback water handling practice has been to flow wellstream fluids (e.g., flowback water) through a gas production unit, or to a direct-fired production separator, for a three-phase separation of the sales gas, the condensate/oil, and the flowback water, at sales gas pressure. Conventional gas production units operate at pressures between 100-1400 psig and typically dump the exiting flowback water directly to atmospheric storage tanks after the sales gas has been separated.
There are several problems inherent in this approach, including inadequate retention times in the production separator and the flashing off of gas at the condensate and flowback atmospheric tanks resulting from the large pressure drop (e.g., from about 1400 psig to atmospheric pressure of about 15 psi). Flash gases vented at flowback water and condensate storage tanks pose severe dangers of fires/explosions and environmental hazards of substantial amounts of HAPs emissions including VOCs.
In addition to environmental concerns, the costs associated with operating oil and gas wells are always a concern. In the oil and gas industry, some of the initial costs to an operator begin in the construction of the location and the cost of the surface damages paid out to the owners of the surface property. Due to the rising cost of land, some of these initial expenditures are cut back by merely building the oil and gas location to accommodate the particular equipment that will be necessary in the drilling, completion, and production portions of the well. This has caused a dramatic size decrease in the oil and gas pads being built today. The size of the equipment necessary to complete a well no longer constitutes the size in which the pad will be constructed, but rather, the size of the pad constructed will determine the amount and size of the equipment that can be used during the process of oil and gas exploration.
Due to these land conservation efforts and environmental concerns, a need exists for a separator assembly constructed in a space saving technique, which is transportable, and which is capable of handling large volumes of flowback water inherent in hydraulic fracturing operations, and to carry out a four-stage separation process without allowing substantial amounts of VOCs and HAPs to be emitted to the atmosphere. It is to such an apparatus that the inventive concepts disclosed herein are directed.